Problem with two displace (bottom of the sea and continents)
If I get your question right, since displacement can have many levels...(positive and negative, it depend on the "midlevel" you choose), you could displace with an image that could displace both sea depth and land height (mountains): in the image below, with midlevel 0.5, white is above Z=0, black is below Z=0, gray is in the middle (to get Z=0 you need a perfect middle grey, so RGB(0.5,0.5,0.5))



Can this concept work in your setup?

Since you unaccepted this answer, I guess it doesn't help you really. It's ok. 

To the benefit of other readers, too, looking for similar results, I'll add a more "realistic" example with a plane (I know a globe is not as simple, but a plane is simpler to understand), though, to better visualize how it can be done: it is just matter to have at least a very good gradient.

Let's build an "Island"...

I took a height map from http://www.reallusion.com/iclone/Help/EarthSculptor/Image/Utilizing_Height_Map_05.jpg

this:



and then


used it as is to create displacement on a plane 
made a colored copy to texture the displacement




UV map of the texture:


then you could also paint over the UV texture to "adapt" it to your land/seascape